Conventionally, this kind of surface illuminating apparatus is well-known as disclosed in, for example, Japanese Utility Model Laid-Open Gazette No. Sho 58-38186. The surface illuminating apparatus, as shown in FIGS. 7 and 8, is so constructed that a light guide plate A of acrylic resin high in transparency is used to form at one surface of the light guide plate A an irregularly reflecting layer B comprising a large number of fine spots for irregularly reflecting the light, the irregularly reflecting layer B is used as the rear surface and a reflecting plate C is layered thereon in a close contact manner, a light diffusion plate D is layered on the surface of the light guide plate A in a close contact manner, and light sources E are disposed at both lateral side surfaces of the light guide plate A, so that the light diffusion plate D is adapted to be illuminated at the entire surface thereof uniformly and brightly.
In the case where the surface illuminating apparatus constructed as the above-mentioned is used for back light of the liquid crystal display, the liquid crystal display, such as words, numerical values, or patterns, displayed throughout an entire liquid crystal display panel, need be recognized with uniform vision of an operator. Therefore, it is required to maintain throughout the entire surface of the liquid display panel a brightness uniformity higher than a predetermined value. Moreover, it is desired that the liquid crystal display, such as, words or numerical values, displayed on the liquid crystal display panel, even when viewed not only at a right angle with respect to the center of the display surface, but also in a range of an angle of visibility shifted at a predetermined angle (generally 60.degree.) vertically and horizontally with respect to the center of the display surface, can be recognized with uniform vision.
The conventionally proposed surface illuminating apparatus cannot obtain sufficiently high brightness while keeping a uniform ratio of illuminance throughout the entire surface of the liquid crystal display panel, thereby creating the problem that the liquid crystal display cannot easily be recognized with uniform vision in a range of angles of visibility.
"Butler finish" means a surface finish in which a surface (of a light diffusion plate) is finished to a finely roughened surface like satin (a satin finish). Wrinkle process means a process for providing fine wrinkles and crimps on a surface, specifically a process in which in the manufacturing process of a film which forms a light diffusion plate, the film is inserted between a roller having a roughened surface and a roller having a smooth surface, so as to be toughened at one surface, while it is hot, so that, when it is cooled down, it is so crinkled as to have fine wrinkles and crimps.
These surface finishes and processes are both generally known.
In any case, the light condensing effect is low to thereby remain the problem in that a brightness in the range of an angle of visibility is insufficient, whereby sufficient recognition with uniform vision cannot be obtained over the entire surface of the liquid crystal panel.
In the present invention, the above problem has been researched so as to find that the light diffusion plate (FIG. 7) layered onto the surface of the light guide plate A is defective. The light diffusion plate D, which is so roughened by a wrinkle process so as to form a butler finish to obtain a uniform ratio of illuminance, has light condensing and diffusion properties to a certain extent. However, there is no regular light condensing function, and thus variation often occurs therein and the light condensing operation is not sufficient and the brightness is inadequate. The present invention involves alteration of the light diffusing plate.